Tag helpers registration system

- This registration system accepts tag descriptors that it then uses to feed requests for tag helpers.
- Added some infrastructure pieces that are used to build up valid tag helper descriptors.

#70
if `propertyType` derives from `TagHelperExpression`, want to render / generate / write (pick just one!) a `Func` that looks like `model => model.{what the user typed}` -- not just the attribute value string.  also _never_ want to quote the value string in that case.